With the advent of the computer age, electronic systems have become a staple of modern life. Part and parcel with this spread of technology comes an ever greater drive for more functionality from these electronic systems. A microcosm of this quest for increased functionality is the size and capacity of various semiconductor devices. From the 8 bit microprocessor of the original Apple I, through the 16 bit processors of the original IBM PC AT, to the current day, the processing power of semiconductors has grown while the size of these semiconductors has consistently been reduce. In fact, Moore's law recites that the number of transistors on a given size piece of silicon will double every 18 months.
As semiconductors have evolved into these complex systems utilized in powerful computing architectures, almost universally, the frequency at which these semiconductors devices operate has been increasing. These modern high-performance systems are designed with a target clock frequency which determines the processing speed of the system.
The continuous quest for higher semiconductor performance has pushed clock frequencies deep into the gigahertz frequency range, reducing the period of the clock signal well below a nanosecond. As the working frequency of advanced semiconductor systems has entered the gigahertz domain, testing these high frequency semiconductors for defects is becoming more difficult.
Defects in a semiconductor may be caused by glitches in the fabrication process, and thus may affect a random distribution of semiconductors. To ensure proper operation of these semiconductors these defects must be detected and the defective semiconductors repaired or disposed of. Consequently, after a semiconductor is fabricated, but at some point before it ships to a customer the semiconductor may undergo a testing process in order to discern which semiconductors have defects.
In main, defects present in a semiconductor consist of two types. Structural faults, relating to the design of the semiconductor and functional faults affecting the operation of the components in the semiconductor. Typically, testing for functional faults validates the correct operation of a system, while testing of structural faults targets manufacturing defects.
Structural faults are in turn usually composed of two main types the stuck-at fault and the transition fault. Stuck-at faults affect the logical behavior of the system, while transition faults affect the timing/temporal behavior of the system. The effect of a transition fault at any point in a circuit is that a transition at that point will not reach a flip-flop, primary output or other circuit element within the target clock period of the circuit.
Typically, to detect these transition faults in a semiconductor an ac test is executed. This ac test may provide a clock to the semiconductor near or above the target clock frequency of the semiconductor under test to test the responses of the gates of the semiconductor at these clock speeds. As the speed of semiconductor devices increases, however, this type of testing becomes increasingly problematic.
In the industry today, automatic test equipment is being used to test these semiconductor devices. Thus, to implement full speed ac testing the test equipment must provide a clock signal near or above the target frequency of the semiconductor. To do this usually means an increase in the cost and complexity of the test equipment, as larger power supplies, regulators and more sensitive measuring equipment is utilized with this testing equipment. Additionally, constantly producing these high frequency clock signals while testing many semiconductors generates temperature regulation and power consumption concerns as well.
Typically, then, high frequency semiconductors are tested at less than their target frequency. This is a non ideal solution, however, as many of these transition faults become problematic only at or near the target frequency of the semiconductor.
Thus, a need exists for circuits which can reduce the average frequency of a clock provided to a semiconductor while still providing at least a portion of the clock signal to the semiconductor at a desired clock frequency.